Is the performance of acts of worship a condition for the validity of faith?

Question:
Who is it that holds that the performance of some acts of worship, including some of the pillars of Islam, is not a condition for the validity of faith, but rather a necessary condition for its perfection?

Answer:
This requires a detailed explanation; the obligatory acts are part of the completeness of faith, and leaving off the forbidden acts is part of the completeness of faith; but not every obligatory act leads to disbelief if left off, and not every forbidden act leads to disbelief if committed. This is the view of the Khawarij. The people of the Sunnah and the community have a detailed explanation based on what is known from the chapter on the ruling of the apostate. Thus, the fornicator is an open sinner if he does not consider fornication lawful; his faith is diminished. And the drinker of intoxicants is an open sinner whose faith is diminished, and he does not become an apostate unless he considers it lawful. If he considers fornication lawful and says it is permissible, he has committed disbelief. Or if he says drinking intoxicants is permissible, he has committed disbelief, contrary to the Khawarij. Likewise, leaving off zakah is a major sin, but one does not commit disbelief by it until he considers it lawful. Similarly, if one breaks the fast in Ramadan intentionally, he does not commit disbelief; he is a sinner according to the people of the Sunnah and the community.
This is unlike leaving off the prayer: there is a difference of opinion among scholars if one does not deny its obligation: does he commit disbelief or not by leaving it off? There are two views; and the most correct is that he commits disbelief by leaving it off alone, even if he does not deny its obligation. We ask Allah for safety.